There have been multiple firmware upgrades over the years. These are usually downloaded over the internet and installed by the end user. Mainly the addition of new DSP features.
The latest upgrade is a hardware upgrade to Mk 2 status. The existing analog module is replaced with the Mk 2 version. The Mk 2 uses ESS 9038PRO DAC chips, instead of ESS 9018 previously.
The "white paper" can be accessed using this link:
https://www.weiss.ch/files/downloads/dac501-dac502/White-paper-on-the-DAC50x-4ch.pdf